The present invention relates to a laser machining method and a laser machining apparatus by which a laser beam is positioned by two mirrors whose axes of rotation are arranged in a transverse manner and made to enter a converging lens, and a work is machined with converged (focused) laser beam.
FIG. 6 shows the optical system of the head unit in a conventional laser machining device.
A first laser beam 20, which is a P-polarized beam, output from a laser oscillator (not shown) is reflected from a fixed mirror 4 through a polarized beam mixer 30, and then from a mirror 1a (X mirror) and a mirror 1b (Y mirror) through an fθ lens 3 onto a printed circuit board 100. A second laser beam 10, which is an S-polarized beam, output from another laser oscillator (not shown) after being reflected by a fixed mirror 5 and further reflected by mirrors 2a and 2b, is reflected by the polarized beam mixer 30 and then reflected from the mirrors 1a and 1b through the fθ lens 3 onto the printed circuit board 100. The machining range of the first laser beam 20 is an area 101 and that of the second laser beam 10 is an area 102. The mirrors 1a and 1b, the fixed mirrors 4 and 5, the mirrors 2a and 2b, the polarized beam mixer 30 and the fθ lens 3 are arranged in a head unit Z indicated by surrounding in the drawing by one-dot chain lines (Patent Document 1).
Next, the mirrors 1a and 1b will be described in more detail.
FIGS. 7A to 7C show the relationship between the mirrors 1a and 1b and the fθ lens 3 wherein 7A is a top view of the optical system of the machining unit; 7B is a right side view of 7A; and 7C is a unfolded optical diagram in the proceeding direction of the laser beam (the direction indicated by a one-dot chain line).
If the reflective faces of the mirrors 1a and 1b are flat, the laser beam 20 whose section is round, for instance, converges to a focal position Fa (Fa is the designed focal position) of the fθ lens 3 whose focal length is f. Here, in FIGS. 7A to 7C, 1aa denotes the rotation axis of an actuator which rotates the mirror 1a and 1bb donates the rotation axis of an actuator which rotates the mirror 1b. 
Incidentally, though the diameter (spot diameter) of the laser beam 20 before and after the converging position Fa (the Z direction here) is greater than the spot diameter in the converging position Fa, the circularity of the beam is secured. For instance, where the beam diameter suitable for machining a hole of the 50 μm in diameter is 30 mm, circularity of 95% or more is achieved in a range of about ±50 μm with reference to the focal position Fa.
In this laser machining device, since a positioning range 102 determined by the second set of mirrors (the mirrors 2a and 2b) can be substantially overlapped with a positioning range 101 determined by the first set of mirrors (the mirrors 1a and 1b), the efficiency of machining can be enhanced by positioning the first laser beam 20 and the second laser beam 10 (see Patent Document 1).    Patent Document 1: JP-A-249364/2004